Optical recording medium having dual information surfaces

ABSTRACT

The optical recording medium of this invention includes: a first substrate having a first information surface; a semitransparent reflection film formed on the first information surface of the first substrate; a second substrate having a second information surface; a reflection film formed on the second information surface of the second substrate; and an adhesive layer for adhering the first substrate and the second substrate so that the first information surface and the second information surface face each other, wherein the thickness of the first substrate is 0.56 mm or more, the thickness of the adhesive layer is 30 μm or more, and the total thickness of the first substrate and the adhesive layer is 0.68 mm or less.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an optical recording mediumwhere a light beam is focused on the recording medium and information isreproduced by detecting light reflected from the recording medium.Particularly, the present invention relates to an optical recordingmedium having dual information surfaces.

[0003] 2. Description of the Related Art

[0004] In recent years, optical recording media have become more andmore important as a means for storing sound information data, imageinformation data, and various information apparatus data because theycan store and reproduce a large amount of data. There are stillrequirements for further increasing the capacity of the opticalrecording media and reducing the size of optical recording/reproducingapparatuses. In order to satisfy these requirements, the storagecapacity of the optical recording media needs to be further increased.

[0005] Compact disks (CDs) having one information surface, for example,are known as a conventional read-only optical recording medium. The CDincludes a spiral information track composed of convex and concaveportions (pits) formed on a surface of a disk-shaped resin substratewith a thickness of 1.2 mm. A reflection film made of aluminum and thelike and a protection film are formed on the resultant informationsurface of the substrate by sputtering and the like. An identificationlabel is then printed on the protection film.

[0006] The storage capacity of such a CD is small because the CD hasonly one information surface. In order to increase the storage capacity,a recording medium where two disks are adhered together, such as a 5″magneto-optical (MO) disk, has been commercialized. The 5″ MO disk isclassified into two types; a disk having one information surface(one-sided disk) and a disk having two information surfaces(double-sided disk). The one-sided disk includes a spiral guide groovecomposed of convex and concave portions formed on a surface of adisk-shaped resin substrate with a thickness of 1.2 mm. A dielectricfilm, a magneto-optical recording material film, another dielectricfilm, and a reflection film made of aluminum and the like are formed inthis order on the resultant information surface of the substrate bysputtering and the like. Another resin substrate with a thickness of 1.2mm is then adhered to the reflection film. The double-sided diskincludes a spiral guide groove composed of convex and concave portionsformed on a surface of a disk-shaped resin substrate with a thickness of1.2 mm. A dielectric film, a magneto-optical recording material film,another dielectric film, and a reflection film made of aluminum and thelike are formed in this order on the resultant information surface ofthe substrate by sputtering and the like. The thus-fabricated disk isadhered with another disk fabricated in the same manner. Conventional 5″MO disk recording/reproducing apparatuses are designed to receive boththe one-sided disk and the double-sided disk to accomplish the recordingand reproduction. The user can select the one-sided disk wheninformation to be recorded is small or the double-sided disk when it islarge. The 5″ MO disk apparatuses are generally provided with only oneoptical head. Accordingly, when the double-sided disk is used, the diskneeds to be taken out and turned over to continue the recording orreproduction.

[0007] In general, the information density of a recording medium isdetermined by the pitch of an information track and the informationdensity in the tracking direction, i.e., the information linear density.In order to increase the information density of the recording medium,the track pitch should be small, while the linear density should belarge. In recent years, there have been studies to increase the densityof the optical recording medium by reducing the thickness of thesubstrate to 0.6 mm, for example, to reduce the aberration of a lightbeam passing through the substrate due to a tilt of the disk.

[0008] However, the above conventional techniques have the followingproblems. In the case of the conventional double-sided optical recordingmedium, if both the top and bottom surfaces of the recording medium areilluminated with light beams so as to record information or reproducerecorded information, little space is left on the surfaces of therecording medium for printing an identification label. This isinconvenient for handling the recording medium. Also, when theconventional double-sided optical recording medium is used for anoptical reproduction apparatus having only one optical head, the opticalrecording medium needs to be taken out from the apparatus and turnedover to continue the reproduction. In order to continue the reproductionautomatically, two optical heads disposed above and below the recordingmedium are required. An apparatus having two optical heads is large insize and its cost is high.

[0009] Another problem is that when a new optical recording mediumthinner than the conventional optical recording media is commercializedto increase the density of the recording medium, such a new opticalrecording medium is not compatible with the conventionalrecording/reproduction apparatus.

SUMMARY OF THE INVENTION

[0010] The optical recording medium of this invention includes: a firstsubstrate having a first information surface; a semitransparentreflection film formed on the first information surface of the firstsubstrate; a second substrate having a second information surface; areflection film formed on the second information surface of the secondsubstrate; and an adhesive layer for adhering the first substrate andthe second substrate so that the first information surface and thesecond information surface face each other, wherein the thickness of thefirst substrate is 0.56 mm or more, the thickness of the adhesive layeris 30 μm or more, and the total thickness of the first substrate and theadhesive layer is 0.68 mm or less.

[0011] In one embodiment, the thickness of the first substrate is in therange of 0.56 mm to 0.6 mm, and the thickness of the adhesive layer isin the range of 40 μm to 60 μm.

[0012] In another embodiment, a recording material film is formed on thereflection film for the second substrate for recording and reproducinginformation.

[0013] In still another embodiment, the recording material film is madeof a phase-change type recording material.

[0014] In still another embodiment, a label is formed on a surface ofthe second substrate.

[0015] In still another embodiment, a spiral track is formed on each ofthe first and second substrates, and the direction of the formation ofthe spiral track on the first substrate is the same as the direction ofthe formation of the spiral track on the second substrate when thespiral tracks are viewed from the side of a surface of the firstsubstrate opposite to the first information surface.

[0016] In still another embodiment, a spiral track is formed on each ofthe first and second substrates, and the direction of the formation ofthe spiral track on the first substrate is reverse to the direction ofthe formation of the spiral track on the second substrate when thespiral tracks are viewed from the side of a surface of the firstsubstrate opposite to the first information surface.

[0017] Alternatively, the optical recording medium of this inventionincludes: a first substrate having a first information surface; asemitransparent reflection film formed on the first information surfaceof the first substrate; a second substrate having a second informationsurface; a reflection film formed on the second information surface ofthe second substrate; and an adhesive layer for adhering the firstsubstrate and the second substrate so that the first information surfaceof the first substrate faces a surface of the second substrate oppositeto the second information surface, wherein the thickness of the firstsubstrate is substantially the same as the thickness of the secondsubstrate.

[0018] Alternatively, the optical recording medium of this inventionincludes: a first substrate having a first information surface; asemitransparent reflection film formed on the first information surfaceof the first substrate; a second substrate having a second informationsurface; a reflection film formed on the second information surface ofthe second substrate; an adhesive layer for adhering the first substrateand the second substrate so that the first information surface of thefirst substrate faces a surface of the second substrate opposite to thesecond information surface; and a label formed on the reflection filmfor the second substrate, wherein the thickness of the first substrateis substantially the same as the thickness of the second substrate.

[0019] Thus, the invention described herein makes possible theadvantages of (1) providing an optical recording medium having dualinformation surfaces where a label can be easily printed on a surface ofthe recording medium, information can be automatically reproduced by useof one optical head, and the compatibility with an optical recordingmedium having one information surface can be secured, and (2) providingan optical recording medium which includes a substrate with a thicknessdifferent from the conventional standard but is compatible withconventional apparatuses so that information stored in the opticalrecording medium can be reproduced.

[0020] These and other advantages of the present invention will becomeapparent to those skilled in the art upon reading and understanding thefollowing detailed description with reference to the accompanyingfigures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a sectional view of an optical recording medium ofExample 1 according to the present invention.

[0022]FIGS. 2A and 2B are views showing optical paths of reflected lightbeams when information recorded on a first information surface and asecond information surface, respectively, is reproduced in Example 1.

[0023]FIG. 3 is a sectional view of an optical recording medium ofExample 2 according to the present invention.

[0024]FIGS. 4A and 4B are views showing optical paths of reflected lightbeams when information recorded on a first information surface and asecond information surface, respectively, is reproduced in Example 2.

[0025]FIG. 5 is a sectional view of an optical recording medium ofExample 3 according to the present invention.

[0026]FIG. 6 is an enlarged sectional view showing a second optical diskof the optical recording medium of Example 3.

[0027]FIGS. 7A to 7C are graphs showing the measurement results ofjitters obtained from trial-manufactured optical recording media.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] The present invention will be described by way of examples withreference to the accompanying drawings.

EXAMPLE 1

[0029]FIG. 1 is a schematic sectional view showing an optical recordingmedium 101 of Example 1 according to the present invention. The opticalrecording medium 101 is a one-side read type recording medium composedof a first optical disk 102 and a second optical disk 103 adhered toeach other. Such an optical recording medium can provide excellentperformance as a digital video disk (DVD).

[0030] The first optical disk 102 includes a disk-shaped first substrate104 having a first information surface 105 where a spiral informationtrack composed of convex and concave portions (pits) is formed. Asemi-transparent first reflection film 106 is formed on the firstinformation surface 105 of the first substrate 104 by sputtering and thelike. The semitransparent first reflection film 106 is made of gold(Au), aluminum (Al), and the like, for example. The first reflectionfilm 106 is supposed to have a property of reflecting part of laserlight for reproduction while transmitting the remaining, as will bedescribed later in detail. In order to realize this property, not onlythe selection of an appropriate material for the reflection film, butalso the adjustment of the thickness thereof to an appropriate range arerequired. The thickness of the first reflection film 106 is preferablyin the range of 5 to 20 nm. In Example 1, the thickness of thereflection film 106 is 10 nm.

[0031] The second optical disk 103 includes a disk-shaped secondsubstrate 107 having a second information surface 108 where a spiralinformation track composed of convex and concave portions (pits) isformed. A second reflection film 109 is formed on the second informationsurface 108 of the second substrate 107 by sputtering and the like. Thesecond reflection film 109 is made of aluminum (Al) and the like.

[0032] Information is recorded on the first and second informationsurfaces 105 and 108 with high density, i.e., a track pitch of about0.74 μm and a minimum pit length of about 0.4 μm. The thickness of thesecond reflection film 109 is smaller than the lengths of the pitsformed on the second information surface 108 so that the pits recordedon the second information surface 108 can be transferred well to thesecond reflection film 109. Specifically, the thickness of the secondreflection film 109 is preferably in the range of 30 to 150 nm. InExample 1, the thickness of the second reflection film 109 is 50 nm.

[0033] As shown in FIG. 1, an adhesive layer 110 is formed between thefirst optical disk 102 and the second optical disk 103 for adhering thetwo optical disks. The adhesive layer 110 is made of an acrylicultraviolet (UV)-curable material, for example. Such a UV-curablematerial is applied to at least one of the optical disks 102 and 103.Then, the two optical disks 102 and 103 are put in contact with eachother via the UV-curable material, and illuminated with a UV ray to curethe UV-curable material and thus to adhere the two optical disks. Otherthermosetting adhesives may also be used instead of the UV-curablematerial.

[0034] A label 111 is attached to the surface of the second optical disk103. A hole 112 (inner diameter: 15 mm) is formed in the center of theoptical recording medium 101 for mounting the optical recording medium101 on a driving motor.

[0035] Now, referring to FIGS. 2A and 2B, the reproduction ofinformation recorded on the first and second information surfaces 105and 108 will be described.

[0036]FIG. 2A shows the case where information recorded on the firstinformation surface 105 is read, while FIG. 2B shows the case whereinformation recorded on the second information surface 108 is read. Aparallel light beam 201 is converged by a focusing lens 202 andilluminates the optical recording medium 101 from the side opposite tothe side of the label 111, i.e., from the side of the first substrate104.

[0037] The focusing lens 202 is designed to be used for opticalrecording media having a substrate with a thickness of 0.6 mm.Accordingly, a conventional-disk having one information surface whichhas a substrate with a thickness of 0.6 mm is also applicable to thefocusing lens 202.

[0038] Referring to FIG. 2A, when information recorded on the firstinformation surface 105 is reproduced, the light beam 201 is controlledto be focused on the first information surface 105 by a known focusingcontrol technique. A reflected light beam 203 reflected from the firstreflection film 106 is detected by an optical detector 205 via asplitter 204. Thus, the information is read. Referring to FIG. 2B, wheninformation recorded on the second information surface 108 isreproduced, the light beam 201 is controlled to be focused on the secondreflection film 109 by the focusing control technique. A reflected lightbeam 206 reflected from the second reflection film 109 is detected bythe optical detector 205, and thus the information is read. For thereproduction of information recorded in the optical recording medium101, the wavelength of the light beam 201 needs to be 650 nm, and thenumeral aperture (NA) of the focusing lens 202 needs to be about 0.6.

[0039] When information recorded on the first information surface 105 isreproduced, as shown in FIG. 2A, the reflected light beams 203 and 206pass through the focusing lens 202 and are received by the opticaldetector 205. However, the size of the spot of the light beam 201 formedon the second reflection film 109 is in the order of several tens ofmicrometers, which is considerably larger than the track pitch (0.8 μm)and the minimum pit length (0.5 μm). Thus, a plurality of pits areilluminated with the light beam 201. Accordingly, the reflected lightbeam 206 hardly includes an individual pit information component, andhas a substantially constant light amount as if it is reflected from asurface where no pit is formed. Further, only part of the reflectedlight beam 206 passes through the focusing lens 202, and the part of thereflected light beam 206 which has passed through the focusing lens 202is not made parallel. Accordingly, the light amount of the part of thereflected light beam 206 which has reached the optical detector 205 issmall. Thus, the pit information detected by the optical detector 205 ismostly composed of components modulated by the pits recorded on thefirst information surface 105.

[0040] When information recorded on the second information surface 108is reproduced, as shown in FIG. 2B, the reflected light beams 203 and206 pass through the focusing lens 202 and are received by the opticaldetector 205. As in the above case, however, the size of the spot of thelight beam 201 formed on the first reflection film 106 is in the orderof several tens of micrometers, which is considerably large andilluminates a plurality of pits. Accordingly, the reflected light beam203 hardly includes individual pit information components. Further,since part of the reflected light beam 203 which has passed through thefocusing lens 202 is not made parallel, the light amount of the part ofthe reflected light beam 203 which has reached the optical detector 205is small. Thus, the pit information detected by the optical detector 205is mostly composed of components modulated by the pits recorded on thesecond information surface 108.

[0041] Next, the relationship between the reflectances of the first andsecond reflection films 106 and 109 will be described. In the case whereinformation recorded on the first information surface 105 is read, asthe reflectance of the first reflection film 106 is higher, the lightamount of the reflected light beam 203 is larger and the quality of theresultant reproduced signal is better. However, in the case whereinformation recorded on the second information surface 108 is read, asthe reflectance of the first reflection film 106 is higher, the lightamount of the light beam 201 passing through the first reflection film106 is smaller. Since the reflected light beam 206 reflected from thesecond reflection film 109 passes through the first reflection film 106again, the light amount of the reflected light beam 206 is furtherreduced at the reading of the information recorded on the secondinformation surface 108. In other words, in the case where informationrecorded on the second information surface 108 is read, the light beam201 passes through the first substrate 104, the first reflection film106, and the adhesive layer 110 to reach the second reflection film 109.The reflected light beam 206 reflected from the second reflection film109 then passes through the adhesive layer 110, the first reflectionfilm 106, and the first substrate 104 again. The light thus passesthrough the first reflection film 106 twice. Accordingly, if thereflectance of the first reflection film 106 is high, the light amountof the reflected light beam 206 becomes small at the reading of theinformation recorded on the second information surface 108. In order toovercome this problem, in the optical recording medium 101 of Example 1according to the present invention, the reflectances of the first andsecond reflection films 106 and 109 are set so that a light amount P₂ ofthe reflected light beam 206 at the reading of information recorded onthe second information surface 108 is substantially the same as a lightamount P₁ of the reflected light beam 203 at the reading of informationrecorded on the first information surface 105. In this case, therelationship is expressed by k₂=k₁/(1−k₁)² where k₁ is the reflectanceof the first reflection film 106 and k₂ is the reflectance of the secondreflection film 109.

[0042] The above expression is obtained in the following manner. Thelight amount P₁ of the reflected light beam 203 at the reading ofinformation recorded on the first information surface 105 is expressedby P₁=P₀×k₁. The light amount P₂ of the reflected light beam 206 at thereading of information recorded on the second information surface 108 isexpressed by P₂=P₀×k₂(1−k₁)². Since P₁=P₂, the above expression isobtained. The reflectances k₁ and k₂ represent the percentage of thereflected light amount with respect to the incident light amount.Specifically, in the optical recording medium 101 of Example 1, thereflectance of the first reflection film 106 is in the range of 20 to35%, while the reflectance of the second reflection film 109 is 60% ormore. The reflectance of the second reflection film 109 is preferably ashigh as possible. However, in order to realize a reflectance closer to100% using an inexpensive material such as aluminum, the film thicknessneeds to be about 0.6 to 0.8 μm. Since a high-density optical disk has apit length of about 0.5 μm, a reflection film as thick as 0.6 to 0.8 μmlowers the level of the transfer of the information surface onto thereflection film. In order to prevent the lowering of the level of thetransfer, the thickness of the second reflection film 109 is made equalto or less than the pit length of the second information surface 108,i.e., 0.5 μm, and thus the reflectance is made 60% or more.

[0043] Next, the aberration of the light beam 201 focused by thefocusing lens 202 will be described. In the optical recording medium 101of Example 1, the optical path length of the light beam 201 wheninformation recorded on the first information surface 105 is read andthat when information recorded on the second information surface 108 isread are different by the total of the thickness of the first reflectionfilm 106 and a thickness t₀ of the adhesive layer 110. Since thethickness of the first reflection film 106 is 0.5 μm or less in theoptical recording medium 101 of Example 1, it can be neglected. When theoptical path length, i.e., the thickness through which the light beampasses varies, the light beam 201 focused by the focusing lens 202generates an aberration. The aberration increases in proportion to aboutthe fourth power of the NA of the focusing lens 202.

[0044] The relationship between the focusing lens 202 and a thickness t₁of the first substrate 104 will be described. Herein, the thickness t₁of the first substrate 104 is considered to include the thickness of thefirst reflection film 106 because the thickness of the first reflectionfilm 106 is negligible in comparison with the thickness t₁ of the firstsubstrate 104 and the thickness t₀ of the adhesive layer 110.

[0045] In general, the focusing lens 202 is designed in consideration ofthe thickness of a substrate of an optical disk. When the thickness of asubstrate of an optical disk having one information surface is 0.6 mm,the focusing lens 202 is designed based on the thickness of thesubstrate of 0.6 mm. When the optical recording medium 101 having thefirst substrate 104 with the thickness t₁ of 0.6 mm is reproduced by useof this focusing lens 202, no problem arises when information recordedon the first information surface 105 is read. However, when informationrecorded on the second information surface 108 is read, the thickness ofthe adhesive layer 110 is added to the thickness of the first substrate104. That is, if the thickness t₀ of the adhesive layer 110 is 40 μm, 40μm is added to the thickness t₁ of the first substrate 104, 0.6 mm. Thisis substantially equal to the case where a substrate with a thickness of0.64 mm is used. Accordingly, the aberration increases when informationrecorded on the second information surface 108 is read. This lowers thequality of the reproduced signal. In order to overcome this problem,when the focusing lens 202 designed for an optical recording mediumhaving a 0.6 mm thick substrate is used, the thickness t₁ of the firstsubstrate 104 is made 0.58 mm as a standard. Then, the thickness of asubstrate of an optical recording medium having dual informationsurfaces is made slightly thinner than that of a substrate of an opticalrecording medium having one information surface. As a result, thethickness of the substrate is 0.58 mm when information recorded on thefirst information surface 105 is read, while it is 0.62 mm wheninformation recorded on the second information surface 108 is read. Inthe latter case, the thickness of the substrate is equal to the distancebetween the incident surface of the first substrate 104 and the secondinformation surface 108.

[0046] The differences between these thicknesses and the design value0.6 mm for the focusing lens 202 are both 20 μm. Thus, substantially thesame quality of reproduced signals can be obtained from the firstinformation surface 105 and the second information surface 108.Naturally, variations are generated in the thickness t₁ of the firstsubstrate 104 and the thickness t₀ of the adhesive layer 110 in thefabrication process. With the above setting, however, the allowances ofthese variations are widened.

[0047] The relationship between the thickness t₁ of the first substrate104 and the thickness t₀ of the adhesive layer 110 will be described inmore detail with reference to FIGS. 7A to 7C. FIGS. 7A to 7C are graphsshowing the measurement results obtained from various trial-manufacturedoptical recording media. The X-axis of these graphs is the distancebetween the incident surface of the first substrate 104 and theinformation surface, and the Y-axis is the jitter of the reproducedsignal. The jitter is the value obtained by dividing the standarddeviation value of the time-axis variation in the reproduced signal bythe period of the channel clock. In FIG. 7A, four types of the firstoptical disks 102 having the first substrate 104 with the thickness t₁of 0.56 mm, 0.57 mm, 0.62 mm, and 0.63 mm were trial-manufactured. Eachof these first optical disks 102 was adhered with the second opticaldisk 103 via the adhesive layer 110 with a thickness of 30 μm. Using theresultant optical recording media, reproduction of information wasconducted, and the jitters were measured. The reference numeral 71 showsthe jitters obtained when information recorded on the first informationsurface 105 is reproduced, while the reference numeral 72 shows thejitters obtained when information recorded on the second informationsurface 108 is reproduced. In FIG. 7B, four types of the first opticaldisks 102 having the first substrate 104 with the thickness t₁ of 0.56mm, 0.57 mm, 0.58 mm, and 0.61 mm were trial-manufactured. Each of thesefirst optical disks 102 was adhered with the second optical disk 103 viathe adhesive layer 110 with a thickness of 40 μm. Using the resultantoptical recording media, reproduction of information was conducted, andthe jitters were measured. The reference numeral 73 shows the jittersobtained when information recorded on the first information surface 105is reproduced, while the reference numeral 74 shows the jitters obtainedwhen information recorded on the second information surface 108 isreproduced. In FIG. 7C, three types of the first optical disks 102having the first substrate 104 with the thickness t₁ of 0.61 mm, 0.62mm, and 0.63 mm were trial-manufactured. Each of these first opticaldisks 102 was adhered with the second optical disk 103 via the adhesivelayer 110 with a thickness of 50 μm. Using the resultant opticalrecording media, reproduction of information was conducted, and thejitters were measured. The reference numeral 75 shows the jittersobtained when information recorded on the first information surface 105is reproduced, while the reference numeral 76 shows the jitters obtainedwhen information recorded on the second information surface 108 isreproduced.

[0048] In general, when information recorded on a disk is reproduced,defocusing and off-tracking arise due to deflection and decentering ofthe disk, vibration and shock applied to the apparatus from outside, andthe like. These deteriorate the jitter of the reproduced signal. Thejitter of the reproduced signal is also deteriorated when the disk andthe optical axis of the light beam are inclined against each other. Thiswarp of the disk varies depending on a change of the environmentalconditions such as humidity. A variation among optical heads in thefabrication process and a variation of each optical head with timeshould also be considered. Accordingly, in order to reproduceinformation recorded on a disk with high reliability, the jitter of areproduced signal is about 10% at maximum in consideration of thedeterioration of the jitter due to various factors described above.

[0049] In comparison between FIGS. 7A and 7B, the following areobserved. When the thickness t₀ of the adhesive layer 110 is 30 μm, thejitter of the reproduced signal obtained from the first informationsurface little changes. The value is kept at and around 9.5%,irrespective of the change in the thickness t₁ of the first substrate104 from 0.56 mm to 0.63 mm. On the contrary, when the thickness t₀ ofthe adhesive layer 110 is 40 μm, the jitter of the reproduced signalobtained from the first information surface is higher as the thicknesst₁ of the first substrate 104 is thinner. This indicates that, when thethickness t₀ of the adhesive layer 110 is as thin as 30 μm, theinfluence of a leak signal from the second information surface isgreater than the influence of the aberration due to the change in thethickness t₁ of the first substrate 104. It is therefore expected that,if the thickness t₀ of the adhesive layer 110 is smaller than 30 μm, theleak signal from the second information surface will be greater and thequality of the reproduced signal will be eminently reduced. Accordingly,the thickness t₀ of the adhesive layer 110 should be 30 μm or more.

[0050] From FIG. 7B, it is observed that the jitter of the reproducedsignal obtained from the first information surface 105 starts increasingsharply when the thickness t₁ of the first substrate 104 is in the rangeof 0.58 to 0.56 mm. This is because, when the thickness t₀ of theadhesive layer 110 is 40 μm, the influence of the aberration due to thechange in the thickness t₁ of the first substrate 104 becomes greaterthan the influence of a leak signal from the second information surface.The jitter of the reproduced signal changes substantially parabolicallywith the change in the thickness t₁ of the first substrate 104. It istherefore expected that, if the thickness t₁ of the first substrate 104is smaller than 0.56 mm, the jitter of the reproduced signal willsharply increase. Accordingly, the thickness t₁ of the first substrate104 should be 0.56 mm or more.

[0051] The total of the thickness t₁ of the first substrate 104 and thethickness t₀ of the adhesive layer 110, i.e., (t₀+t₁) is the thicknessof a substrate existing when information recorded on the secondinformation surface 108 is reproduced. From FIG. 7C, it is observed thatthe jitter of the reproduced signal starts increasing sharply when thethickness (t₀+t₁) is in the range of 0.66 to 0.68 mm. The jitter changessubstantially parabolically with the change in the thickness t₁ of thefirst substrate 104. It is therefore expected that, if the thickness(t₀+t₁) of the substrate is 0.69 mm, the jitter of the reproduced signalwill exceed 10%. Accordingly, in order to obtain a jitter of thereproduced signal of 10% or less, the total of the thickness t₁ of thefirst substrate 104 and the thickness t₀ of the adhesive layer 110,i.e., (t₀+t₁) should be 0.68 mm or less.

[0052] The above values are very strict. In order to secure thereliability of the device, severe examination is required for eachcomponent of the device. Since the allowances are too narrow to allowmass production, the cost of each device becomes high. The allowancesshould be widened in order to manufacture the devices easily. This pointwill be described in more detail as follows.

[0053] When information recorded on the first information surface 105 isreproduced, as the thickness t₀ of the adhesive layer 110 is larger, theinfluence of a leak signal from the second information surface 108 issmaller. As is observed from the comparison between FIGS. 7A and 7B, thethickness t₀ of the adhesive layer 110 is desirably 40 μm or more wheninformation recorded on the first information surface 105 is reproduced.Further, when the thickness t₁ of the first substrate 104 is 0.56 mm ormore, the jitter of the reproduced signal can be as small as 8%.

[0054] As is observed from FIG. 7A, when the distance between thesurface of the first substrate 104 and the second information surface108 is 0.66 mm, the jitter of the reproduced signal is 7.5%. From FIG.7B, when the above distance is 0.65 mm, the jitter is 6.6%. From FIG.7C, when the above distance is 0.66 mm, 0.67 mm, and 0.68 mm, the jitteris 7%, 7.8%, and 8.8%, respectively. Thus, when the above distanceexceeds 0.66 mm, the jitter of the reproduced signal sharply increases.Accordingly, the thickness of the substrate existing when informationrecorded on the second information surface 108 is reproduced, i.e., thetotal of the thickness t₁ of the first substrate 104 and the thicknesst₀ of the adhesive layer 110 is desirably 66 mm or less.

[0055] When the focusing lens 202 is designed for an optical recordingmedium having a 0.6 mm thick substrate, the thickness of the substratepreferably varies with 0.6 mm as the center of the variation.Accordingly, when the thickness of the first substrate 104 is 0.56 mm ormore, it can be defined as 0.58 mm±0.02 mm. Therefore, in order toobtain the total of the thickness of the first substrate 104 and thethickness of the adhesive layer 110 of 0.66 mm, the thickness of theadhesive layer 110 should be 60 μm or less.

[0056] From the above description, it is understood that, by setting thethickness t₀ of the adhesive layer 110 in the range of 40 to 60 μm andthe thickness t₁ of the first substrate 104 in the range of 0.56 mm to0.6 mm, the jitters of the reproduced signals obtained from the firstand second information surfaces 105 and 108 are both low, and thusreproduced signals with excellent quality can be obtained.

[0057] Now, the direction of the spiral tracks of the first and secondoptical disks 102 and 103 will be described. For example, when thespiral track of the first optical disk 102 is formed from the inner sideto the outer side and the spiral track of the second optical disk 103 isalso formed from the inner side to the outer side, interactivereproduction can be realized by using one optical head for thereproduction from the dual information surfaces. For example, a programof a game having a plurality of branches may be recorded on the dualinformation surfaces separately. In the game, upon receipt of branchinginstruction, the program can instantaneously move from the firstinformation surface 105 to the second information surface 108 or fromthe second information surface 108 to the first information surface 105by focus jumping.

[0058] Alternatively, when the spiral track of the first optical disk102 is formed from the inner side to the outer side, and the spiraltrack of the second optical disk 103 is formed from the outer side tothe inner side, continuous reproduction can be easily realized by usingone optical head for the reproduction from the dual informationsurfaces. That is, information is first reproduced from the firstinformation surface 105 by moving the optical head from the inner sideto the outer side of the disk. When the optical head reaches theoutermost side, the focusing is instantaneously jumped from the firstinformation surface 105 to the second information surface 108. Then,information recorded on the second information surface 108 is reproducedfrom the outer side to the inner side. This procedure allows forlong-time continuous reproduction of a movie and the like. Such anoptical recording medium including the first and second optical disks ofwhich spiral tracks are formed in directions reverse to each other isobtained in the following manner: At the cutting of the original disks,signals are recorded in the first optical disk 102 by moving the opticalhead from the inner side to the outer side of the disk. When signals areto be recorded in the second disk 103, the disk is rotated reversely,and the optical head is moved from the outer side to the inner side ofthe disk.

[0059] Thus, in the optical recording medium in Example 1, the first andsecond optical disks are adhered via an adhesive having a predeterminedthickness. Information recorded on both the first and second informationsurfaces is reproduced by illuminating the surfaces with a light beamfrom one side of the optical recording medium. Thus, a label can beattached to the other side. Further, since information recorded on boththe first and second information surfaces is reproduced only by changingthe position of the focusing point by use of one optical head,interactive reproduction or long-time continuous reproduction of a movieis possible. This also reduces the cost of an recording/reproducingapparatus. Moreover, since the thicknesses of the first and secondoptical disks are the same, these optical disks little change in shapewith the change in humidity. This facilitates the adhesion of theseoptical disks, and thus lowers the cost of the disks.

EXAMPLE 2

[0060] In Example 2, an optical recording medium which can be used fordifferent types of optical recording/reproducing apparatuses designedfor optical recording media having substrates with different thicknesseswill be described.

[0061]FIG. 3 shows a schematic sectional view of an optical recordingmedium from which information can be read by both an apparatus designedfor a recording medium with a 1.2 mm thick substrate and an apparatusdesigned for a recording medium with a 0.6 mm thick substrate.

[0062] An optical recording medium 301 of Example 2 is composed of afirst optical disk 302 and a second optical disk 303 adhered to eachother. The same information is stored in the first and second opticaldisks 302 and 303. The first optical disk 302 includes a disk-shapedfirst substrate 304 with a thickness of 0.6 mm having a firstinformation surface 305 where a spiral information track composed ofconvex and concave portions (pits) is formed. A semitransparent firstreflection film 306 is formed on the first information surface 305 ofthe first substrate 304 by sputtering and the like. The second opticaldisk 303 stores completely the same information as that in the firstoptical disk 302 in the same way. The second optical disk 303 includes adisk-shaped second substrate 307 with a thickness of 0.6 mm having asecond information surface 308 where a spiral information track composedof convex and concave portions (pits) is formed. A second reflectionfilm 309 of aluminum and the like is formed on the second informationsurface 308 of the second substrate 307 by sputtering and the like. Thereference numeral 310 denotes an adhesive layer made of a UV-curablematerial for adhering the first and second optical disks 302 and 303.The reference numeral 311 denotes a label for identifying the opticalrecording medium. The reference numeral 312 denotes a hole for mountingthe optical recording medium 301 on an optical recording/reproducingapparatus.

[0063] The reproduction of information recorded on the first and secondinformation surfaces 305 and 308 will be described with reference toFIGS. 4A and 4B. FIG. 4A shows the case where information recorded onthe first information surface 305 is read by use of an apparatusdesigned for an optical recording medium having a 0.6 mm thicksubstrate. FIG. 4A is basically the same as FIG. 2A. That is, a parallellight beam 201 is converged by a focusing lens 202 designed for a 0.6 mmthick substrate and illuminates the optical recording medium 301 fromthe side of the first substrate 304. The light beam 201 is partlyreflected from the first reflection film 306 and a reflected light beam203 is detected by an optical detector 205 via a splitter 204. Thus, theinformation is read.

[0064]FIG. 4B shows the case where information recorded on the secondinformation surface 308 is read by use of an apparatus designed for anoptical recording medium having a 1.2 mm thick substrate. Referring toFIG. 4B, when information recorded on the second information surface 308is reproduced, a parallel light beam 401 is converged by a focusing lens402 designed for a 1.2 mm thick substrate and illuminates the opticalrecording medium 301 from the side of the first substrate 304. The lightbeam 401 passes through the first substrate 304, the first reflectionfilm 306, the adhesive layer 310, and the second substrate 307 andreaches the second information surface 308. The light beam 401 is partlyreflected from the second reflection film 309, and a reflected lightbeam 406 passes through the second substrate 307, the adhesive layer310, the first reflection film 306, the first substrate 304, and thefocusing lens 402. The reflected light beam 406 is then detected by anoptical detector 405 via a splitter 404. Thus, the information is read.

[0065] As shown in FIG. 4A, when information recorded on the firstinformation surface 305 is reproduced, the reflected light beams 203 and206 pass through the focusing lens 202 and are received by the opticaldetector 205. However, the size of the spot of the light beam 201 formedon the second reflection film 309 is as large as 1 mm or more, and thusa plurality of pits are illuminated with the light beam 201. Also, thereflected light beam 206 which has passed through the focusing lens 202is not made parallel. Accordingly, the light amount of the reflectedlight beam 206 which has reached the optical detector 205 is extremelysmall. Thus, pit information components obtained from the secondinformation surface 308 are hardly detected by the optical detector 205.

[0066] As shown in FIG. 4B, as in the above case, when informationrecorded on the second information surface 308 is reproduced, thereflected light beams 403 and 406 pass through the focusing lens 402 andare received by the optical detector 405. However, the size of the spotof the light beam 401 formed on the first reflection film 306 is aslarge as 1 mm or more, and thus a plurality of pits are illuminated withthe light beam 401. Also, the reflected light beam 406 which has passedthrough the focusing lens 402 is not made parallel. Accordingly, pitinformation components obtained from the first information surface 305are hardly detected by the optical detector 405.

[0067] The relationship between the reflectances of the first and secondreflection films 306 and 309 in the optical recording medium 301 ofExample 2 is basically the same as that of the optical recording medium101 of Example 1. In the optical recording medium 301, however,information recorded on the second information surface 308 is notrequired to be transferred to the second reflection film 309. Thisenables the second reflection film 309 to be thickened, and thus areflectance of 90% or more can be obtained.

[0068] As described above, when the optical recording medium 301 shownin FIG. 3 is used as an apparatus designed for a 1.2 mm thick substrate,the light beam 401 passes through the first and second substrates 304and 307, the first reflection film 306, and the adhesive layer 310. Whenthe thicknesses of the first and second substrates 304 and 307 are 0.6mm, the total thickness exceeds 1.2 mm by the thickness of the adhesivelayer 310, though the thickness of the first reflection film 306 isnegligible. This causes aberration. In order to solve this problem, thethickness of the adhesive layer 310 is preferably several tens ofmicrometers or less. Alternatively, the thickness of the secondsubstrate 307 may be thinned by the thickness of the adhesive layer 310.

[0069] The first and second information surfaces 305 and 308 may havedifferent formats from each other. For example, information may berecorded on the second information surface 308 with the format ofconventional CDs so that the information can be reproduced bywidely-available CD players. In general, the density of CDs is low andthe capacity thereof is only a quarter or so of that of the optical diskaccording to the present invention. Accordingly, for example, while theentire movie may be recorded on the first information surface 305, anedited version of the movie shortened by cutting part thereof may berecorded on the second information surface 308. In this case, since alight beam with a wavelength of 780 nm is used for reproduction fromCDs, the first reflection film 306 should have the optical property ofreflecting a 650 nm light beam and transmitting a 780 nm light beam.This increases the reflected light amount and thus the S/N ratio of theresultant reproduced signal.

[0070] As described above, in Example 2, since the same information isrecorded in the first and second optical disks, the same information canbe read by both an apparatus designed for an optical recording mediumhaving a 1.2 mm thick substrate and an apparatus designed for an opticalrecording medium having a 0.6 mm thick substrate. Also, since both thefirst and second information surfaces are illuminated with a light beamfrom one side of the optical recording medium, a label can be attachedto the other side.

EXAMPLE 3

[0071] In Example 3, an optical recording medium having a firstinformation surface for reproduction only and a second informationsurface for recording and reproduction will be described. Thereproduction or recording of information is conducted by illuminatingthe optical recording medium with a light beam from only one side.

[0072]FIG. 5 is an exaggerated sectional view of an optical recordingmedium 501 of Example 3. The optical recording medium 501 is composed ofa first optical disk 502 for reproduction only and a second optical disk503 for recording and reproduction adhered to each other. The firstoptical disk 502 includes a disk-shaped substrate 504 with a thicknessof 0.6 mm having a first information surface 505 where a spiralinformation track composed of convex and concave portions (pits) isformed. A semitransparent reflection film 506 is formed on the firstinformation surface 505 of the substrate 504 by sputtering and the like.The second optical disk 503 includes a substrate with a thickness of0.58 mm having a second information surface where a spiral informationtrack composed of minute convex and concave portions (grooves) isformed. The reference numeral 510 denotes an adhesive layer for adheringthe first and second optical disks 502 and 503. The reference numeral511 denotes a label for identifying the optical recording medium. Thereference numeral 512 denotes a hole for mounting the optical recordingmedium 501 on an optical recording/reproducing apparatus.

[0073] In the optical recording medium 501 shown in FIG. 5, as in theoptical recording medium 101 shown in FIG. 1, the first informationsurface 505 for reproduction only and the second information surface forrecording and reproduction are adhered via the adhesive layer 510 sothat they are apart from each other by about 40 μm. The opticalrecording medium 501 is illuminated with a light beam from the side ofthe first optical disk 502.

[0074] Referring to FIGS. 5 and 6, the second optical disk 503 will bedescribed. FIG. 6 is an enlarged exaggerated sectional view obtained bycutting the second optical disk 503 in a radial direction. A groovetrack 602 having convex and concave portions is formed on one surface ofa substrate 601 of the second optical disk 503. Then, a reflection film603 made of aluminum and the like, a dielectric film 604 made of SiO₂and the like, a recording material film 605, and another dielectric film606 are formed consecutively in this order by sputtering and the like.The reflection film 603 is disposed to enhance the sensitivity andprotect the recording material film 605 from thermal shock byfacilitating heat radiation. The recording material film 605 is formed,for example, by sputtering a phase-change type recording materialcontaining tellurium (Te), antimony (Sb), and germanium (Ge) as maincomponents. The dielectric films 604 and 606 are formed to protect therecording material film 605 from humidity or thermal shock. Thesedielectric films can be omitted.

[0075] The phase-change type recording material becomes crystalline whengradually cooled after heating and becomes amorphous when abruptlycooled after melting. This property is used in the phase-change typedisk, where the crystalline state and the amorphous state of thephase-change type recording material is reversibly changed to eachother, so that information can be overwritten repeatedly as is done onmagnetic disks such as floppy disks and hard disks. Information isrecorded on the phase-change type disk as follows. The disk is rotatedat a predetermined speed. While the tracking is controlled so as tolocate a light beam along the groove track, the intensity of the lightbeam is changed between the strong amorphous level and the weakcrystalline level depending on the signal to be recorded. For example,in the case where the recording is conducted so that the recording markis in the amorphous state, a light beam with a light amount large enoughto melt the film is radiated so as to form a mark in the amorphous stateon the film. On the contrary, during the period when no mark is to beformed, a light beam with a light amount small enough to prevent thefilm from melting is radiated, so as to crystallize the position of thefilm. At this time, therefore, the position of the film is crystallizedregardless of the previous state of the position, amorphous orcrystalline. Thus, even the position of the film where information hasbeen recorded is overwritten. The reproduction of the informationrecorded on the phase-change type disk is conducted based on theprinciple that the reflectances of the amorphous state and thecrystalline state are different from each other. For example, the diskis illuminated with a constant weak light beam, a reflected light beamfrom the disk is detected by an optical detector, and a detectedvariation in the reflected light amount is used to reproduceinformation.

[0076] As described above, the optical recording medium 501 of Example 3is constructed to receive a light beam from the side of the firstoptical disk 502. The reason is as follows. Information is recorded inthe second optical disk 503 for recording and reproduction by use ofheat obtained by absorbing the light beam. Thus, in order to conduct therecording using a light beam with a small light amount, about 60% of thelight beam needs to be absorbed by the second optical disk. Accordingly,when the reflectance is about 20%, the transmittance is as small as 20%.If the optical recording medium is constructed to receive a light beamfrom the side of the second optical disk 503, in reverse to the case ofExample 3, the reflected light amount required at the reading ofinformation recorded on the first information surface 505 will becomeextremely small. For example, the reflected light amount will be only 4%of the incident light amount after passing through the second opticaldisk 503 twice even if the reflectance of the reflection film 506 is100%. The above trouble is avoided in the case of the optical recordingmedium 501 according to the present invention, which receives a lightbeam from the side of the first optical disk 502. The absorbance of thesecond optical disk 503 can be as large as 60%, while the reflectancethereof can be 40%. Thus, when the reflectance of the reflection film506 is 20%, for example, a reflected light amount of about 20% of theincident light amount is obtained in the case where information recordedon the first information surface 505 is read, and a reflected lightamount of about 26% of the incident light amount is obtained in the casewhere information recorded in the second optical disk 503 is read. Wheninformation recorded in the first optical disk 502 for reproduction onlyis reproduced, the incident light beam is greatly modulated by pitsformed on the first information surface 505. Accordingly, a reproducedsignal with sufficiently high quality can be obtained even when thereflectance of the first reflection film 506 is as low as 20%.

[0077] As described above, since the optical recording medium 501 ofExample 3 is constructed to receive a light beam from the side of thefirst optical disk 502 for reproduction only, reproduction from both thefirst and second optical disks 502 and 503 can be conducted with highreliability.

[0078] Thus, in Example 3, the first and second optical disks 502 and503 are adhered with an adhesion with a predetermined thickness, and theinformation surfaces of the two optical disks are illuminated with alight beam from one side of the optical recording medium. Accordingly, alabel can be attached to the other side of the optical recording medium.Also, since information recorded on the dual information surfaces can bereproduced only by changing the position of the focusing point of alight beam by use of one optical head, interactive reproduction ispossible, and the cost of the optical recording/reproducing apparatus isreduced. Further, the optical disk for reproduction only and the opticaldisk for recording and reproduction are combined to form an opticalrecording medium. Accordingly, for example, information recorded in theoptical disk for reproduction only may be processed and the processedinformation may be recorded in the optical disk for recording andreproduction. This makes it easy to handle the information since relatedinformation is stored in the same optical recording medium. Since thethicknesses of the first and second optical disks are the same, theseoptical disks little change in shape with the change in humidity. Thisfacilitates the adhesion of the optical disks, and thus lowers the costof the optical recording/reproducing medium.

[0079] Incidentally, a recording material film similar to the recordingmaterial film 605 used in Example 3 may be formed on the secondinformation surface 308 of the optical recording medium of Example 2.Such a recording material film should be formed between the secondinformation surface 308 and the second reflection film 309 of the secondoptical disk 303.

[0080] Thus, according to the present invention, the first optical diskincluding the semitransparent reflection film formed on the firstinformation surface where information is recorded and the second opticaldisk including the reflection film formed on the second informationsurface where information is recorded are adhered with a transparentadhesive so that the information surfaces are closer to each other.Accordingly, the information recorded on the dual information surfacescan be read by illuminating the surfaces with a light beam radiated fromone side of the optical recording medium. Thus, nearly double the amountof information can be consecutively reproduced. Since a label can beattached to the other side of the optical recording medium, theidentification of the optical recording medium is easy.

[0081] The thicknesses of the first substrate and the adhesive layer areset at predetermined values. Accordingly, the jitters of reproducedsignals obtained from the first and second information surfaces are low,and thus reproduced signals with high quality can be obtained.

[0082] Alternatively, according to the present invention, thethicknesses of the first and second substrates are made substantiallythe same, and these substrates are adhered to each other so that thefirst information surface of the first substrate faces the surface ofthe second substrate opposite to the second information surface. Such anoptical recording medium can be used for both an apparatus designed foran optical recording medium with a 1.2 mm thick substrate and anapparatus designed for an optical recording medium with a 0.6 mm thicksubstrate.

[0083] Alternatively, according to the present invention, the opticaldisk for reproduction only and the optical disk for recording andreproduction are adhered to each other. Information recorded in theoptical disk for reproduction only may be processed, for example, andthe processed information may be recorded in the optical disk forrecording and reproduction. This makes it easy to handle the informationsince related information is stored in one optical recording medium.

[0084] Various other modifications will be apparent to and can bereadily made by those skilled in the art without departing from thescope and spirit of this invention. Accordingly, it is not intended thatthe scope of the claims appended hereto be limited to the description asset forth herein, but rather that the claims be broadly construed.

What is claimed is:
 1. An optical recording medium comprising: a firstsubstrate having a first information surface; a semitransparentreflection film formed on the first information surface of the firstsubstrate; a second substrate having a second information surface; areflection film formed on the second information surface of the secondsubstrate; and an adhesive layer for adhering the first substrate andthe second substrate so that the first information surface and thesecond information surface face each other, wherein the thickness of thefirst substrate is 0.56 mm or more, the thickness of the adhesive layeris 30 μm or more, and the total thickness of the first substrate and theadhesive layer is 0.68 mm or less.
 2. An optical recording mediumaccording to claim 1 , wherein the thickness of the first substrate isin the range of 0.56 mm to 0.6 mm, and the thickness of the adhesivelayer is in the range of 40 μm to 60 μm.
 3. An optical recording mediumaccording to claim 1 , wherein a recording material film is formed onthe reflection film for the second substrate for recording andreproducing information.
 4. An optical recording medium according toclaim 3 , wherein the recording material film is made of a phase-changetype recording material.
 5. An optical recording medium according toclaim 1 , wherein a label is formed on a surface of the secondsubstrate.
 6. An optical recording medium according to claim 1 , whereina spiral track is formed on each of the first and second substrates, andthe direction of the formation of the spiral track on the firstsubstrate is the same as the direction of the formation of the spiraltrack on the second substrate when the spiral tracks are viewed from theside of a surface of the first substrate opposite to the firstinformation surface.
 7. An optical recording medium according to claim 1, wherein a spiral track is formed on each of the first and secondsubstrates, and the direction of the formation of the spiral track onthe first substrate is reverse to the direction of the formation of thespiral track on the second substrate when the spiral tracks are viewedfrom the side of a surface of the first substrate opposite to the firstinformation surface.
 8. An optical recording medium comprising: a firstsubstrate having a first information surface; a semitransparentreflection film formed on the first information surface of the firstsubstrate; a second substrate having a second information surface; areflection film formed on the second information surface of the secondsubstrate; and an adhesive layer for adhering the first substrate andthe second substrate so that the first information surface of the firstsubstrate faces a surface of the second substrate opposite to the secondinformation surface, wherein the thickness of the first substrate issubstantially the same as the thickness of the second substrate.
 9. Anoptical recording medium according to claim 8 , wherein a recordingmaterial film is formed between the second information surf ace of thesecond substrate and the reflection film.
 10. An optical recordingmedium according to claim 9 , wherein the recording material film ismade of a phase-change type recording material.
 11. An optical recordingmedium comprising: a first substrate having a first information surface;a semitransparent reflection film formed on the first informationsurface of the first substrate; a second substrate having a secondinformation surface; a reflection film formed on the second informationsurface of the second substrate; an adhesive layer for adhering thefirst substrate and the second substrate so that the first informationsurface of the first substrate faces a surface of the second substrateopposite to the second information surface; and a label formed on thereflection film for the second substrate, wherein the thickness of thefirst substrate is substantially the same as the thickness of the secondsubstrate.
 12. An optical recording medium according to claim 11 ,wherein a recording material film is formed between the secondinformation surface of the second substrate and the reflection film. 13.An optical recording medium according to claim 12 , wherein therecording material film is made of a phase-change type recordingmaterial.